Over the years, an increasing number of environmental sites have been characterized and determined to be polluted with hazardous and toxic chemicals such as halogenated hydrocarbons, benzene, toluene, and xylene. These chemicals pose serious problems to the ecology, especially if they get into drinking water. Therefore, there is an urgent need to identify the location and concentration of such hazardous chemicals.
The identification of chemicals has relied heavily on removal of the chemical samples from a contaminated site and subsequent analysis of the samples using techniques such as gas chromatography and/or mass spectroscopy. These techniques are inherently costly, slow, time consuming, and do not give real time analysis since the chemicals are volatile and their concentrations decrease with time. Hence, the techniques heretofore employed do not give reliable information.
Alternative detection techniques have been utilized based on infrared (IR) fiber optic technology. Silica-based fiber optics is a well-developed technology that has had a major impact on telecommunications, medicine, and industry. Optical fibers fabricated with silica-based glass have achieved the intrinsic attenuation limit of 0.2 dB/km at 1.5 .mu.m. Widespread production of optical fibers from silica-based glasses proves their dominant role in communications for both voice and data; in optical and electro-optical systems in a myriad of fields; and in sensors for medicine, industry, and the military.
The halide glasses are of interest in high energy laser applications because of their low linear and non-linear refractive indices and low dispersion, but are unsuitable for practical handling and use because of their hygroscopicity. Certain halide glasses transmit further in the IR than silica glasses and have potential applications as ultra-low loss fibers but are prone to devitrification and thus cannot be drawn into crystal-free fibers. The heavy metal fluoride glasses exhibit a lesser tendency toward crystallization and higher IR transparency.
Most chemicals possess characteristic vibrational bands in the IR region between about 2-12 micrometers or microns. The chemicals referred to herein include halogenated hydrocarbons, benzene, toluene, and xylene. Since silica-based fibers transmit in the region of up to about 2 .mu.m and halide fibers transmit in the region of up to about 3 .mu.m, it should be apparent that these prior art fibers are incapable of detecting the various chemicals that one may want to detect.
The article by Krska et al. in vol. 47, No. 7, pp. 1484-1487 of the 1993 journal Applied Spectroscopy entitled "The New IR Fiber-Optic Chemical Sensor for in Situ Measurements of Chlorinated Hydrocarbons in Water" discloses an unclad crystalline fiber of silver halide for detecting chlorinated hydrocarbons in water. A low density polyethylene deposited on the fiber concentrates the chlorinated hydrocarbons to facilitate their detection by evanescent wave spectroscopy. Another article by Heo et al in vol. 30, No. 27, dated Sep. 20, 1991, in Applied Optics, entitled "Remote fiber-optic chemical sensing using evanescent-wave interactions in chalcogenide glass fibers" discloses detection of analytes with an unclad chalcogenide fiber.